The present invention relates to a silicone rubber coating composition which firmly adheres to the surface of a variety of substrate materials without the addition of an adhesion promoter. More particularly, the present invention relates to woven fabrics, in particular airbag fabric, which are coated with the silicone rubber coating composition at high production speeds.
Silicone rubbers are widely used commercially for a variety of purposes including electric insulation, seals, gaskets, mold forming materials, coating applications for substrates and the like. It is widely known and accepted in the silicone coating industry that the addition of an adhesion promoter or a coupling agent to a silicone coating composition is necessary in order to achieve adequate adhesion of the silicone coating to a variety of substrates. An adhesion promoter is considered desirable, otherwise a primer has to be utilized. Unfortunately, the use of a primer makes the coating of substrates more complex by adding another process step, and aggravates the working environment with the use of additional organic solvents. Further, primers add to labor costs, which are considerably more than the costs associated with an adhesion promoter being incorporated into the coating composition during the manufacturing process.
Although different substrates may require a different kind of adhesion promoter, the organofunctional silanes are widely used as adhesion promoters in various applications in the art of silicone chemistry. Further, these compounds can be classified into different groups according to their functionality. In this regard, U.S. Pat. No. 5,312,690 discloses the addition of an adhesion promoter also known as a coupling agent to its silicone rubber composition. The adhesion promoter is in the form of an organosilane compound having an isocyanato-containing group-NCO and a hydrolyzable group such as alkoxy groups. By virtue of the addition of this component, the silicone rubber composition as cured firmly adheres to the substrate. U.S. Pat. No. 4,499,234 discloses a curable silicone coating composition comprising, among other components, an adhesion promoter to provide primerless adhesion of the composition to the substrate. U.S. Pat. No. 4,528,353 also discloses silicone compositions containing self-bonding additives of which the amine functional silanes are the most preferred. U.S Pat. Nos. 4,513,115 and 4,539,367 each discloses a silicone rubber composition having an adhesion promoter to facilitate the bonding of the composition to the substrate material.
U.S. Pat. No. 5,258,211 to Momii et al. is indicative of the disadvantages in the existing art. The Momii, '211, reference discloses a coating composition for air bag fabric which combines an organopolysiloxane and a linear organohydrogensiloxane but also further requires an organosilane adhesion promoter, or partial hydrolyzate, thereof. While compositions, such as those disclosed in Momii, and the other prior art described above, which contain adhesion promoters are commonly used in the silicone coating industry, they nevertheless have distinct disadvantages. Adhesion promoters are generally more expensive than the base polymer. As such, the overall costs for manufacturing the coating composition dramatically increases. The addition of the adhesion promoter may also require an extra processing step during the mixing cycle, which further increases the cost of production. Most importantly, it has been found that the addition of an adhesion promoter causes problems during the curing cycle. In this regard, the adhesion promoter can retard the curing time, resulting in a longer processing time for coating compositions containing such adhesion promoters. Extra processing time often results in higher energy costs and overhead and lower productivity. Adhesion promoters may also accelerate the cure, which creates other problems such as scorching of the composition and unpredictable chemical reactions. This results in waste, increased costs and problems in controlling the coating of the silicone rubber composition onto a substrate.